[PATCH] D106301: [OpenMP] Disable trap before unreachable for OpenMP device jobs

[Jon Chesterfield via Phabricator via cfe-commits]

JonChesterfield added a comment.

In D106301#2888170 <https://reviews.llvm.org/D106301#2888170>, @jdoerfert wrote:

> llvm.trap is preserved, thus branches to an llvm.trap are preserved.

That's interesting. Consistent with IR in general,

  template <bool Trap> int test(int x) {
    if (x < 42) {
      return x;
    } else {
      if (Trap)
        __builtin_trap();
      __builtin_unreachable();
    }
  }
  
  extern "C" {
  int trap(int x) { return test<true>(x); }
  int none(int x) { return test<false>(x); }
  }

`=>`

  define i32 @trap(i32 returned %0) {
    %2 = icmp slt i32 %0, 42
    br i1 %2, label %4, label %3
  
  3:                                                ; preds = %1
    tail call void @llvm.trap() #3
    unreachable
  
  4:                                                ; preds = %1
    ret i32 %0
  }
  
  define i32 @none(i32 returned %0)  {
    %2 = icmp slt i32 %0, 42
    tail call void @llvm.assume(i1 %2) #3
    ret i32 %0
  }

So yes, we'll get faster codegen if we are willing to throw away traps followed by unreachable code.

If that's a legitimate transform to do, it seems like something we should do in instcombine, instead of a separate pass. I.e. fold `trap, unreachable` to `unreachable`.

Can we do that instead?


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